1. Field of the Invention
This invention relates to a novel and improved anti-skid control system for motor vehicles, which prevents the wheels of the motor vehicle from skidding during braking operation, and more particularly it pertains to such a system designed to prevent the wheels from locking too early when the vehicle speed is low.
2. Description of the Prior Art
Generally, with an anti-skid control system for motor vehicles, braking control is effected with the aid of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speed sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates manners in which wheel speed Vw and brake pressure Pw are varied in a conventional anti-skid control system. In FIG. 1, the wheel speed Vw is abruptly decelerated as a result of buildup of the brake pressure Pw; and at time t1 when the wheel speed Vw becomes equal to a reference wheel speed which is set up on the basis of the wheel speed Vw in such a manner that it follows the wheel speed Vw with a predetermined relationship therewith, the brake pressure Pw is reduced so that the wheel speed Vw is changed from decleration to acceleration. At time t2 when there occurs a low peak of the wheel speed Vw where the latter is changed from deceleration to acceleration, the brake pressure Pw is kept constant; subsequently at time t3 when a high peak of the wheel speed Vw occurs, the brake pressure Pw is again increased. During the buildup of the brake pressure Pw which is caused at the time t3, brake pressure increasing and holding modes are repeated alternately and mincingly so that the brake pressure Pw is increased gradually; and at time t4 when the wheel speed Vw becomes equal to the aforementioned reference wheel speed like at the time t1, the brake pressure is reduced again, and similar control procedures are repeated.
With the above-described conventional anti-skid control system wherein buildup of the brake pressure Pw is started when a high peak of the wheel speed Vw occurs, however, it is likely that when the vehicle speed is low, the wheel speed Vw does not recover up to the level of the vehicle speed and rather drops further away from the level of the vehicle speed. In such a case, if the speed change gear of the motor vehicle is operated, then the deceleration of the wheel speed tends to be abruptly changed due to vibration of the speed change gear or differential gear, so that abrupt buildup control of the brake pressure Pw is produced, thus causing the wheels to be locked too early at time tn, for example, as shown in FIG. 1.